High-isolation dense wavelength division multiplexer utilizing birefringent plates and a non-linear interferometer

ABSTRACT

The present invention provides a dense wavelength division multiplexer for separating an optical signal into optical channels. The dense wavelength division multiplexer includes at least one birefringent plate for separating one or more of the plurality of optical channels, where the separating mechanism is at least partially based on the polarity of the plurality of optical channels, and a nonlinear interferometer optically coupled to the at least one birefringent plate for introducing a phase difference between at least two of the plurality of optical channels. In a preferred embodiment, the mechanism of separating includes birefringent beam separation and recombination plates, optical rotation elements, a lens and a non-linear interferometer. The present invention provides an ease in alignment and a higher tolerance to drifts due to the increase in the widths of the pass bands and also provides an improved efficiency of separation of channels. It may also be easily modified to perform the add/drop function as it separates channels. The materials required to manufacture and implement the dense wavelength division multiplexer in accordance with the present invention are readily available. The present invention thus does not require special or expensive materials or processes. It is thus cost effective.

The present application is a divisional of U.S. Ser. No. 09/404,005,filed Sep. 23, 1999.

FIELD OF THE INVENTION

The present invention relates to fiber optic networks, and moreparticularly to fiber optic dense wavelength division multiplexers.

BACKGROUND OF THE INVENTION

Fiber optic networks are becoming increasingly popular for datatransmission due to their high speed and high data capacitycapabilities. Multiple wavelengths may be transmitted along the sameoptic fiber. This totality of multiple combined wavelengths comprises asingle transmitted signal. A crucial feature of a fiber optic network isthe separation of the optical signal into its component wavelengths, or“channels”, typically by a wavelength division multiplexer. Thisseparation must occur in order for the exchange of wavelengths betweensignals on “loops” within networks to occur The exchange occurs atconnector points, or points where two or more loops intersect for thepurpose of exchanging wavelengths.

Add/drop systems exist at the connector points for the management of thechannel exchanges. The exchanging of data signals involves theexchanging of matching wavelengths from two different loops within anoptical network. In other words, each signal drops a channel to theother loop while simultaneously adding the matching channel from theother loop.

FIG. 1 illustrates a simplified optical network 100. A fiber opticnetwork 100 could comprise a main loop 150 which connects primarylocations, such as San Francisco and New York. In-between the primarylocations is a local loop 110 which connects with loop 150 at connectorpoint 140. Thus, if local loop 110 is Sacramento, wavelengths at SanFrancisco are multiplexed into an optical signal which will travel fromSan Francisco, add and drop channels with Sacramento's signal atconnector point 140, and the new signal will travel forward to New York.Within loop 110, optical signals would be transmitted to variouslocations within its loop, servicing the Sacramento area. Localreceivers (not shown) would reside at various points within the localloop 110 to convert the optical signals into the electrical signals inthe appropriate protocol format.

The separation of an optical signal into its component channels istypically performed by a dense wavelength division multiplexer. FIG. 2illustrates add/drop systems 200 and 210 with dense wavelength divisionmultiplexers 220 and 230. An optical signal from Loop 110 (λ₁-λ_(n))enters its add/drop system 200 at node A (240). The signal is separatedinto its component channels by the dense wavelength division multiplexer220. Each channel is then outputted to its own path 250-1 through 250-n.For example, λ₁ would travel along path 250-1, λ₂ would travel alongpath 250-2, etc. In the same manner, the signal from Loop 150(λ₁′-λ_(n)′) enters its add/drop system 210 via node C (270). The signalis separated into its component channels by the wavelength divisionmultiplexer 230. Each channel is then outputted via its own path 280-1through 280-n. For example, λ₁′ would travel along path 280-1, λ₂′ wouldtravel along path 280-2, etc.

In the performance of an add/drop function, for example, λ₁, istransferred from path 250-1 to path 280-1. It is combined with theothers of Loop 150's channels into a single new optical signal by thedense wavelength division multiplexer 230. The new signal is thenreturned to Loop 150 via node D (290). At the same time, λ₁′ istransferred from path 280-1 to path 250-1. It is combined with theothers of Loop 110's channels into a single optical signal by the densewavelength division multiplexer 220. This new signal is then returned toLoop 110 via node B (260). In this manner, from Loop 110's frame ofreference, channel λ₁ of its own signal is dropped to Loop 150 whilechannel λ₁′ of the signal from Loop 150 is added to form part of its newsignal. The opposite is true from Loop 150's frame of reference. This isthe add/drop function.

Conventional methods used by wavelength division multiplexers inseparating an optical signal into its component channels include the useof filters and fiber gratings as separators. A “separator,” as the termis used in this specification, is an integrated collection of opticalcomponents functioning as a unit which separates one or more channelsfrom an optical signal. Filters allow a target channel to pass throughwhile redirecting all other channels. Fiber gratings target a channel tobe reflected while all other channels pass through. Both filters andfiber gratings are well known in the art and will not be discussed infurther detail here.

A problem with the conventional separators is the precision required ofa device for transmitting a signal into an optic fiber. A signalentering a wavelength division multiplexer must conform to a set of verynarrow pass bands. FIG. 3 shows a sample spectrum curve 310 comprised ofnumerous channels as it enters a dense wavelength division multiplexer.The pass bands 320 of the channels are very narrow. Ideally, the curvewould be a square wave. A narrow pass band is problematic because, dueto the physical limitations and temperature sensitivity of signal sourcelaser devices, they never emit light exactly at the center wavelength ofan optical filter. The difference between the actual wavelength and thewavelength at the center of the pass band is called the “offset.” Theamount of offset or change in offset (“drift”) ideally should not belarger than the width of the pass bands. Otherwise, crosstalk betweenchannels will be too large. Crosstalk occurs when one channel or part ofa channel appears as noise on another channel adjacent to it. Since thesignals resulting from the conventional wavelength division multiplexerconfigurations have narrow pass bands, the signal source devices(“transmitter”), such as lasers or the like, must be of a high precisionso that offset or drift is limited to the width of the pass bands. Thishigh precision is difficult to accomplish. Signal transmitting devicesof high precision are available but are very expensive. Also, the signaltransmitting devices must be aligned individually for each separator,which is time intensive.

Therefore, there exists a need for a separation mechanism which wouldallow a wavelength division multiplexer to have a greater tolerance forwavelength offset and drift and a greater ease of alignment than isrealized by conventional separators. The present invention addressessuch a need.

SUMMARY OF THE INVENTION

The present invention provides a dense wavelength division multiplexerfor separating an optical signal into optical channels. The densewavelength division multiplexer includes at least one birefringent platefor separating one or more of the plurality of optical channels, wherethe separating mechanism is at least partially based on the polarity ofthe plurality of optical channels, and a nonlinear interferometeroptically coupled to the at least one birefringent plate for introducinga phase difference between at least two of the plurality of opticalchannels. In a preferred embodiment, the mechanism of separatingincludes birefringent beam separation and recombination plates, opticalrotation elements, a lens and a non-linear interferometer. The presentinvention provides an ease in alignment and a higher tolerance to driftsdue to the increase in the widths of the pass bands and also provides animproved efficiency of separation of channels. It may also be easilymodified to perform the add/drop function as it separates channels. Thematerials required to manufacture and implement the dense wavelengthdivision multiplexer in accordance with the present invention arereadily available. The present invention thus does not require specialor expensive materials or processes. It is thus cost effective.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a simplified optical network.

FIG. 2 is an illustration of conventional add/drop systems and densewavelength division multiplexers.

FIG. 3 is a graph of two sample spectrum curves, each comprised ofseveral channels, as they enter, respectively, a conventional wavelengthdivision multiplexer and a dense wavelength division multiplexer inaccordance with the present invention.

FIG. 4 is an illustration of an embodiment of a separator utilizing apolarization beam splitter and a non-linear interferometer.

FIG. 5 is a side view of a first embodiment of the separator inaccordance with the present invention.

FIG. 6 is an end view of the fiber configuration of the input and outputfibers of the separator of the present invention.

FIG. 7 is an illustration of a preferred embodiment of a nonlinearinterferometer used with the separator of the present invention.

FIG. 8 is a sequence of cross sections through the first embodiment ofthe separator of the present invention illustrating the locations andpolarization states of fiber images created by the light of signals andsub-signals of odd channels.

FIG. 9 is a sequence of cross sections through the first embodiment ofthe separator of the present invention illustrating the locations andpolarization states of fiber images created by the light of signals andsub-signals of even channels.

FIG. 10 is a flow chart illustrating a preferred embodiment of a methodfor separating an optical signal in accordance with the presentinvention.

FIG. 11 is a functional signal routing diagram for the separator of thepresent invention illustrating its functioning as a 2×2 switch.

FIG. 12 is a side view of a second embodiment of the separator inaccordance with the present invention.

FIG. 13 is a side view of a third embodiment of the separator inaccordance with the present invention.

FIG. 14 is a simple block diagram of a wavelength division multiplexerwith a multistage parallel cascade configuration of separators inaccordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to an improvement in the separationmechanism to be used in a dense wavelength division multiplexer. Thefollowing description is presented to enable one of ordinary skill inthe art to make and use the invention and is provided in the context ofa patent application and its requirements. Various modifications to thepreferred embodiment will be readily apparent to those skilled in theart and the generic principles herein may be applied to otherembodiments. Thus, the present invention is not intended to be limitedto the embodiment shown but is to be accorded the widest scopeconsistent with the principles and features described herein.

A separator utilizing a polarization beam splitter and a non-linearinterferometer which is more advanced than, conventional separators isdisclosed in a co-pending U.S. Patent Application entitled “Non-LinearInterferometer for Fiber Optic Wavelength Division multiplexer Utilizinga Phase Differential Method of Wavelength Separation,” Ser. No.09/247,253, filed on Feb. 2, 1999.

FIG. 4 illustrates a top view of a preferred embodiment of the separator1000 disclosed in U.S. patent application Ser. No. 09/247,253. Thisseparator 1000 separates the signal into two sets of channels. Theseparator 1000 comprises an optical fiber 1010 for inputting an opticalsignal and optical fibers 1020 and 1030 for outputting optical signals.As the signal leaves the optic fiber 1010, it diverges. A lens 1050collimates the signal and directs it towards a beam splitter 1070 whichdecomposes the signal based upon its polarity. This decomposition takesplace at a plane 1075 of the beam splitter 1070. The component(P-component) of the input signal polarized within the plane defined bythe input signal's direction of travel and a line perpendicular to plane1075 passes through beam splitter 1070 towards an interferometer 800Bhaving waveplates 890B and 895B. The component (s-component) of theinput signal polarized parallel to plane 1075 is reflected towards aninterferometer 800A having waveplates 890A and 895A. The interferometers800A and 800B introduce phase differences between the even and oddchannels of the signals. The even channels travel to one output fiber1030 through lens 1055, and the odd channels travel to the other outputfiber 1020.

This separator 1000 has advantages over conventional separators in termsof increased widths of the pass bands and isolation bands and greaterease of alignment. Although the separator 1000 is useful for its statedpurpose, it may be limited, in some cases, by the properties of thepolarization beam splitter 1070 used therein. A perfect polarizationbeam splitter will separate an incident unpolarized light beam into twoplane polarized component light beams with mutually perpendicularpolarization orientations such that each component beam comprises 100%of the light of one polarization orientation and none of the light ofthe other orientation. However, in real beam splitters, which can neverbe perfect, there is always a small amount of leakage of light rays ofone polarization orientation into the pathway nominally comprised onlyof light with the other polarization orientation. Because of thisleakage, there will be imperfect isolation of one set of signals fromanother in the separator 1000. The separator in accordance with thepresent invention improves upon the separator illustrated in FIG. 4.

To more particularly describe the features of the present invention,please refer to FIGS. 5 through 14 in conjunction with the discussionbelow.

FIG. 5 shows a side view of a first embodiment of the separator inaccordance with the present invention. The first embodiment of theseparator 500 comprises a four fiber ferrule 515 with four opticalfibers, Fiber A 501, Fiber B 502, Fiber C 503 and Fiber D 504 containedwithin and secured to ferrule 515. FIG. 6 shows an end view of the fiberconfiguration as viewed from the left side of the device of FIG. 5. Fourcollimator lenses 505, 506, 507 and 508 are incorporated into the end offerrule 515 such that each collimator receives light from and directslight to exactly one of the fibers, specifically Fiber A 501, B 502, C503 and D 504, respectively.

Returning to FIG. 5, disposed adjacent to the end of ferrule 515 is afirst birefringent walk-off plate 509 which has the property ofseparating any signal light ray emanating from any of the fibers 501,502, 503 or 504 into two physically separated plane polarized sub-signalrays—one innermost and one outermost sub-signal ray. Because four fibersare contained within ferrule 515, eight separate sub-signals are sodefined and are comprised of four outermost and four innermostsub-signals. The optical axes of birefringent plate 509 are disposedsuch that outermost and innermost sub-signals from both Fiber A 501 andFiber B 502 comprise e-rays and o-rays, respectively, in their traversethrough birefringent plate 509. Conversely, the outermost and innermostsub-signals from both Fiber C 503 and Fiber D 504 comprise o-rays ande-rays, respectively, in their traverse through birefringent plate 509.

In this specification, the polarization plane directions of e-rays ando-rays are referred to as “vertical” and “horizontal,” respectively.Such orientation disposition references are used in a relative senseonly and are made for the clarity of the discussion and the convenienceof the reader and by no means imply restriction of the use of thepresent invention to particular absolute spatial orientations ofelements contained therein or of polarization planes of light rayspropagating therethrough. Various modifications of the embodiments ofthe present invention for use with these and other spatial orientationswill be understood by one of ordinary skill in the art and are withinthe spirit and scope of the present invention.

Disposed adjacent to the first birefringent plate 509 and on the side ofplate 509 opposite to ferrule 515 are both a first half-wave plate 510and a second half-wave plate 511. Half-wave plate 510 is disposed so asto intercept only the two outermost sub-signals arising from Fiber A 501and Fiber B 502. Likewise, half-wave plate 511 is disposed so as tointercept only the two outermost sub-signals arising from Fiber C 503and Fiber D 504. A second birefringent walk-off plate 512 is disposedadjacent to the two half-wave plates 510 and 511 on the side opposite tothe first birefringent plate 509. The thickness of birefringent plate512 is disposed so as to give an offset of one of the rays propagatingtherethrough equivalent to the center-to-center distance between anypair of fibers. A lens or lens assembly 513 is disposed to the side ofthe second birefringent walk-off plate 512 opposite to the half waveplates 510 and 511. Finally, a non-linear interferometer 514 is disposedat the focal point of lens 513 opposite to the birefringent plate 512.

The non-linear interferometer 514 is an instance of an inventiondisclosed in the co-pending U.S. patent application Ser. No. 09/247,253.FIG. 7 illustrates a preferred embodiment of an interferometer describedin patent application Ser. No. 09/247,253. The interferometer 700comprises two parallel glass plates 780A and 780B with a space or cavity710 therebetween. The inside face of the glass plate 780B is coated witha layer of reflective coating 720 with a reflectivity preferably of100%. The inside face of the glass plate 780A is coated with a layer ofreflective coating 740 with a reflectivity preferably of approximately18%. A quarter-wave plate 795 is disposed within the space 710 and aneighth-wave plate 790 is disposed adjacent to plate 780A and external tothe space 710.

When signal 30 enters the interferometer 700, it passes through the 18%reflective coating 740 and a waveplate 795 preferably of λ/4. The λ/4plate 795 introduces an 180° round trip phase change between an o-beamand e-beam of the signal inside the cavity 710, and the external λ/8plate 790 introduces a round trip 90° phase change between the o-beamand e-beam. The waveplate 790, preferably of λ/8, fine tunes the shapeof the signal 30.

Returning to FIG. 5, the non-linear interferometer 514 has the propertysuch that, if the light beam reflected therefrom is an optical signalcomprised of a plurality of channels evenly spaced in wavelength and thelight of each channel is plane polarized, then the light of every secondchannel is reflected with a 90° rotation of its polarization planedirection whilst the light of each remaining channel is reflected withunchanged polarization. In the following discussion, the channels whoselight rays experience the 90° polarization-plane rotation uponinteraction with non-linear interferometer 514 are arbitrarily referredto as even channels and the remaining channels are referred to as oddchannels. The use of such terminology, i.e., “even channels” or “oddchannels”, in this specification is made for the convenience of thereader only and does not imply restriction of the present invention toany particular optical channel wavelength distribution, wavelengthspacing or enumeration scheme. Adaptation of the present invention foruse with any one of numerous optical channel configurations and orsystems will be understood by one of ordinary skill in the art and iswithin the spirit and scope of the present invention. Furthermore, itwill be understood by one of ordinary skill in the art that thenon-linear interferometer comprising this invention may also beconstructed so as to rotate polarization planes of light rays of the“odd channels” instead of those of light rays of the “even channels”without departing from the spirit and scope of the present invention.

The operation of separator 500 is now described with reference to FIGS.8 and 9. FIG. 8 shows a sequence of cross sectional views 801-809 offiber images created by the light of signals and sub-signals of oddchannels within separator 500. FIG. 9 shows a sequence of crosssectional views 901-909 of fiber images created by the light of signalsand sub-signals of even channels within separator 500. Thesecross-sections are all drawn as viewed from the left side of the device500 and are taken at the labeled cross-sectional planes U-U′, V-V′,W-W′, X-X′, and Y-Y′. These cross-sections correspond to locationssimilarly labeled on FIG. 5. The projection of the center of lens513—that is, a point midway between the two lens surfaces and collinearwith the front and rear foci of the lens—along the line connecting thetwo foci and onto each of these cross sections is designated by a plus(“+”) sign. In FIGS. 8 and 9, circles drawn with solid lines are used todenote sub-signals comprised of horizontally polarized light, circlesdrawn with dotted lines are used to denote sub-signals comprised ofvertically polarized light, and concentric solid and dotted circles areused to denote overlapping sub-signals of differing polarization or elsesignals of mixed or random polarization. The sizes of these circles haveno physical significance. All sub-signal light is reflected in thenon-linear interferometer 514 of separator 500 so as to make onecomplete forward and one complete return traverse through separator 500.Therefore, each cross-section of sub-signal fiber images is shown twice,and heavy arrows indicate the relative sequence of images defined bylight propagating through separator 500.

The paths of signals and sub-signals of odd channels are now describedwith reference to FIG. 8. As seen in cross section U-U′ 801 of FIG. 8,signals emanating from each of the four fibers—Fiber A 501, Fiber B 502,Fiber C 503 and Fiber D 504—are comprised of randomly polarized light.After emanating from one of the four fibers and passing through one ofthe collimator lenses 505-508, each light signal enters and passesthrough the first birefringent plate 509 which divides it intophysically separated sub-signal components whose light rays arepolarized horizontally and vertically, respectively. In FIG. 8,sub-signal A 810, sub-signal B 812, sub-signal C 814 and sub-signal D816 represent the horizontally polarized sub-signal light emanating,respectively, from Fiber A 501, Fiber B 502, Fiber C 503 and Fiber D504. Likewise, sub-signal A′ 811, sub-signal B′ 813, sub-signal C′ 815and sub-signal D′ 817 represent the vertically polarized sub-signallight emitting, respectively, from Fiber A 501, Fiber B 502, Fiber C 503and Fiber D 504.

The four vertically polarized sub-signals A′ 811, B′ 813, C′ 815 and D′817 all comprise e-rays during their traverse through the firstbirefringent plate 509. Therefore, as shown in cross-section V-V′ 802 ofFIG. 8, sub-signals 811, 813, 815 and 817 are all shifted in a firstdirection perpendicular to the fiber axes with respect to the matchinghorizontally polarized sub-signals, A 810, B 812, C 814 and D 816,respectively. After passing the first birefringent plate 509, the fouroutermost sub-signals A′ 811, B′ 813, C 814 and D 816 pass through oneof the two 90° half-wave plates, 510 and 511, and therefore eachsub-signal incurs a 90° rotation of the polarization plane direction ofits light rays. Thus, as shown in cross section W-W′ 803 of FIG. 8, thepolarization plane directions of light rays of sub-signals A′ 811 and B′813 change from vertical to horizontal whilst those of light rays ofsub-signals C 814 and D 816 change from horizontal to vertical.

After passing the positions of the half-wave plates 510 and 511, allsub-signals enter and pass through the second birefringent walk-offplate 512. The four sub-signals comprised of vertically polarized light,C′ 815, D′ 817, C 814 and D 816, traverse birefringent plate 512 ase-rays and are thus deflected. Simultaneously, the four sub-signalscomprised of horizontally polarized light, A′ 811, B′ 813, A 810 and B812, traverse birefringent plate 512 as undeflected o-rays. Thethickness of birefringent plate 512 is chosen such that the lateraldeflection of e-rays upon traversing therethrough is in the firstdirection and is exactly equal to the inter-fiber center-to-centerdistance. For this reason, after passing through birefringent plate 512,the two sub-signal images C′ 815 and C 814 become superimposed on thesub-signal images A′ 811 and A 810, respectively and the two sub-signalimages D′ 817 and D 816 become superimposed on the sub-signal images B′813 and B 812, respectively. This, superimposition of sub-signals isshown in cross section X-X′ 804 of FIG. 8.

The line connecting the two foci of lens 513 is indicated by a plussymbol where it intersects each of the cross sections of FIGS. 8 and 9.As indicated in FIG. 8 (or FIG. 9), the lens 513 is positioned suchthat, between second birefringent plate 512 and lens 513, to the fourpairs of superimposed sub-signals are symmetrically disposed about thisline connecting the two foci of lens 513. This symmetry is such that,between second birefringent plate 512 and lens 513, the four pairs ofsuperimposed sub-signals define two mutually orthogonal planes of mirrorsymmetry whose intersection is the line connecting the two foci of lens513. In cross section X-X′ of FIG. 8 (or FIG. 9), the four sub-signalsA′ 811, C′ 815, B′ 813 and D′ 817 are mirrored by sub-signals A 810, C814, B 812 and D 816 through the first such, or horizontal, mirrorplane. The four sub-signals A′ 811, C′ 815, A 810 and C 814 are mirroredby sub-signals B′ 813, D′ 817, B 812 and D 816 through the second such,or vertical, mirror plane.

After exiting plate 512, each pair of superimposed sub-signals, A′ 811and C′ 815, A 810 and C 814, B′ 813 and D′ 817, and B 812 and D 816travels along its own path with the two sub-signals comprising each pairremaining superimposed, one upon the other and with all pathwaysparallel to the line connecting the two foci of lens 513. These fourpairs of sub-signals travel to and through the lens 513, which bringsthem all to a common focal point within the non-linear interferometer514 as shown in cross-section Y-Y′ 805 of FIG. 8. The non-linearinterferometer 514 reflects these odd-channel sub-signals back alongtheir return paths through separator 500 without a change inpolarization. Thus, the four pairs of sub-signals immediately divergefrom one another after being reflected by the non-linear interferometer514 and pass through lens 513 a second time in the reverse direction.The diverging pathways of the four pairs of returning sub-signals areset parallel to one another by lens 513. Thus, these four pairs ofsubsignals are directed back towards the second birefringent plate 512along pathways which, between the birefringent plate 512 and the lens513, exactly superimpose upon those of forward propagating pairs ofsub-signals and which once again, are parallel to the line connectingthe two foci of lens 513.

Cross section x-x′ 806 of FIG. 8 shows the locations of the pairs ofsuperimposed sub-signal images at their points of return entry intobirefringent plate 512. The focusing and re-collimation of sub-signalimages by lens 513, together with the reflection by the non-linearinterferometer 514, cause the inversion of image positions about thecenter of the lens as projected onto cross-section x-x′ 806. Thisinversion causes interchange of the positions of the various pairs ofsub-signals as projected onto cross-section x-x′ 806. Thus, uponre-entry into plate 512, as shown in cross-section x-x′ 806 of FIG. 8,the location of the returning pair of sub-signal images B 812 and D 816is the same as that of the forward propagating pair of sub-signals A′811 and C′ 815. Likewise, in cross-section x-x′ 806 of FIG. 8, thelocations of returning pairs of sub-signals A 810 and C 814, B′ 813 andD′ 817; and A′ 811 and C′ 815 are identical to those of forwardpropagating pairs of sub-signals B′ 813 and D′ 817, A 810 and C 814, andB 812 and D 816, respectively.

During return passage through the second birefringent plate 512, thesub-signals comprised of vertically polarized light, D 816, C 814, D′817 and C′ 815, pass therethrough as deflected e-rays whilst thosecomprised of horizontally polarized light, B 812, A 810, B′ 813 and A′811, pass therethrough as undeflected o-rays. For this reason, the twosub-signals comprising each pair of superimposed sub-signals becomere-separated one from another upon passing through birefringent plate512 a second time. The deflection of sub-signals D 816, C 814, D′ 817and C′ 815 upon their second traverse through birefringent plate 512 isexactly equal and opposite to their deflection during their firsttraverse through this plate. Therefore, the locations of the images ofthe various sub-signals after the second traverse of these sub-signalsthrough birefringent plate 512 are as shown in cross section w-w′ 807 ofFIG. 8.

After exiting the second birefringent plate 512, the outermost returningsub-signals B 812, A 810, D′ 817 and C′ 815 pass through one of the two90° half-wave plates, 510 and 511, and therefore each incurs a 90°rotation of the polarization plane direction of its light rays. Afterpassing, in the return direction, the positions of the 90° half-waveplates, 510 and 511, the positions and polarization states of thevarious subsignals are as shown in cross section v-v′ 808 of FIG. 8.

Finally, all sub-signals enter the first birefringent walk-off plate 509in the return direction. The sub-signals comprised of verticallypolarized light, B 812, A 810, D 816 and C 814, pass through plate 509as deflected e-rays whilst those comprised of horizontally polarizedlight, B′ 813, A′ 811, D′ 817 and C′ 815, pass through plate 509 asundeflected o-rays. The deflection of sub-signals B 812, A 810, D 816and C 814 during return passage through plate 509 is exactly equal andopposite to the deflection of sub-signals B′ 813, A′ 811, D′ 817 and C′815 during their forward passage through this plate. Therefore, thevertically and horizontally polarized pairs of sub-signals A 810 and A′811, B 812 and B′ 813, C 814 and C′ 815, and D 816 and D′ 817 becomerecombined at the positions of the fiber collimator lenses 505-508. Eachof the collimator lenses focuses the return-path signal impingingthereon into the immediately adjacent fiber. As shown in cross sectionu-u′ 809 of FIG. 8, therefore, the recombined signals are located suchthat the signals originally from Fiber A 501, Fiber B 502, Fiber C 503and Fiber D 504 are directed into Fiber B 502, Fiber A 501, Fiber D 504and Fiber C 503, respectively.

The paths of signals and sub-signals of even channels through theseparator 500 are now described with reference to FIG. 9. Afteremanating from one of the four fibers and passing through one of thecollimator lenses 505-508, signal light enters and passes through thefirst birefringent plate 509 which separates it into physicallyseparated horizontally and vertically polarized sub-signal components.In FIG. 9, sub-signal A 910, sub-signal B 912, sub-signal C 914 andsub-signal D 916 represent the horizontally polarized sub-signal lightemanating, respectively, from Fiber A 501, Fiber B 502, Fiber C 503 andFiber D 504. Likewise, sub-signal A′ 911, sub-signal B′ 913, sub-signalC′ 915 and sub-signal D′ 917 represent the vertically polarizedsub-signal light emanating, respectively, from Fiber A 501, Fiber B 502,Fiber C 503 and Fiber D 504.

The forward propagating pathways of even channel sub-signals throughseparator 500 are identical to those of odd channel sub-signals up untilthey encounter the non-linear interferometer 514 and will not berepeated here. Upon reflection from non-linear interferometer 514,however, the directions of the polarization planes of light of evenchannel sub-signals are all rotated by 90°. As a consequence, afterreflection from non-linear interferometer 514 and embarkation upon theirreturn pathways, the light rays comprising sub-signals A 910, B 912, A′911, and B′ 913, become vertically polarized whilst the light rayscomprising sub-signals C 914, D 916 C′ 915 and D′ 917 becomehorizontally polarized.

Cross section x-x′ 906 of FIG. 9 shows the locations and polarizationstates of even-channel sub-signal images upon their re-entry into secondbirefringent walk-off plate 512. During return passage through thesecond birefringent plate 512, the sub-signals comprised of verticallypolarized light, B 912, A 910, B′ 913 and A′ 911, pass therethrough asdeflected e-rays whilst those comprised of horizontally polarized lightD 916, C 914, D′ 917 and C′ 915 pass therethrough as undeflected o-rays.For this reason, the two sub-signals comprising each pair ofsuperimposed sub-signals become re-separated one from another uponpassing through birefringent plate 512 a second time. The commondeflection of sub-signals B 912, A 910, B′ 913 and A′ 911 upon theirsecond traverse through birefringent plate 512 is exactly equal andopposite to the deflection of sub-signals C 914, D 916, C′ 915 and D′917 during their first traverse through this plate. Therefore, thelocations of the various sub-signals after the second traverse of thesesub-signals through birefringent plate 512 are as shown in cross sectionw-w′ 907 of FIG. 9.

After exiting the second birefringent plate 512, the outermost returningsub-signals D 916, C 914, B′ 913 and A′ 911 pass through one of the two90° half-wave plates, 510 and 511, and therefore each incurs a 90°rotation of the polarization plane direction of its light rays. Afterpassing, in the return direction, the positions of the 90° half-waveplates, 510 and 511, the positions and polarization states of thevarious sub-signals are as shown in cross section v-v′ 908 of FIG. 9.

Finally, all sub-signals enter the first birefringent walk-off plate 509in the return direction. The vertically polarized sub-signals D 916, C914, B 912 and A 910 pass through plate 509 as deflected e-rays whilstthe horizontally polarized sub-signals D′ 917, C′ 915, B′ 913 and A′ 911pass through plate 509 as undeflected o-rays. The common deflection ofsub-signals D 916, C 914, B 912 and A 910 during their return passagethrough plate 509 is exactly equal and opposite to the deflection ofsub-signals D′ 917, C′ 915, B′ 913 and A′ 911 during their forwardpassage through this plate. Therefore, the vertically and horizontallypolarized pairs of sub-signals A 910 and A′ 911, B 912 and B′ 913, C 914and C′ 915, and D 916 and D′ 917 become recombined at the positions ofthe fiber collimator lenses 505-508. Each of the collimator lensesfocuses the return-path signal impinging thereon into the immediatelyadjacent fiber. As shown in cross section u-u′ 909 of FIG. 9, therefore,the recombined signals are located such that the signals originally fromFiber A 501, Fiber B 502, Fiber C 503, and Fiber D 504 are directed intoFiber D 504, Fiber C 503, Fiber B 502 and Fiber A 501, respectively.

FIG. 10 is a flow chart illustrating a preferred embodiment of a methodfor separating an optical signal in accordance with the presentinvention. First, the optical signal is split into a plurality ofoptical channels, via step 1010, using the birefringent plates 509, 512and wave plates 510, 511, as described above. Then, a phase differenceis introduced between at least two of the plurality of optical channels,via step 1020, by the nonlinear interferometer 514. Next, the pluralityof optical channels are reflected, via step 1030, by the nonlinearinterferometer 514. At least two of the reflected optical channels arethen combined, via step 1040, by the birefringent plates 509, 512 andwave plates 510, 511, as described above.

FIG. 11 summarizes the results of the operation of separator 500.Odd-channel signals input to the separator 500 from Fiber A 501 andFiber C 503 are directed to Fiber B 502 and Fiber D 504, respectively,whereas even channel signals input to the separator 500 from Fiber A 501and Fiber C 503 are directed to Fiber D 504 and Fiber B 502,respectively. Similarly, odd-channel signals input to the separator 500from Fiber B 502 and Fiber D 504 are directed to Fiber A 501 and Fiber C503, respectively whereas even channel signals input to the separator500 from Fiber B 502 and Fiber D 504 are directed to Fiber C 503 andFiber A 501, respectively. In this way, the separator 500 functions as adense wavelength division multiplexer or de-multiplexer whichdiscriminates amongst the pathways of odd and even channels.

An added functionality and advantage of this and the other separators ofthe present invention is the ability to perform the add/drop functionwhile also separating the channels. As illustrated in FIG. 10, twosignals, a first signal containing channel signals λ₁-λ_(n) and a secondsignal containing channel signals λ₁′-λ_(n)′, are input into theseparator 500 through Fiber A 501 and Fiber C 503, respectively. Device500 could then drop the even channels of the first signal to the secondsignal while adding the even channels of the second signal to the firstsignal. For instance, as shown in FIG. 10, the output at Fiber B 502would consist of the odd channels (λ₁, λ₃, λ₅ . . . ) from the firstsignal plus the even channels (λ₂′, λ₄′, λ₆′. . . ) from the secondsignal. In like fashion, the output at Fiber D 504 would consist of theodd channels (λ₁′, λ₃′, λ₅′. . . ) from the second signal plus the evenchannels (λ₂, λ₄, λ₆ . . . ) from the first signal.

In the first embodiment of the current invention, separator 500, thesecond birefringent walk-off plate 512 must be of the exact thickness soas to cause a lateral deflection of e-rays equivalent to the inter-fibercenter-to-center distance. This requirement may create difficulties insome circumstances.

A second embodiment of the present invention, which eliminates thisrequirement, is shown in FIG. 12. In the separator 1200 shown in FIG.12, the four-fiber ferrule 515, the fibers 501-504, the four collimatorlenses 505-508, the first birefringent plate 509, the first and second90° half-wave plates 510 and 512, the lens 513 and the non-linearinterferometer 514 are common to the separator 500. However, in theseparator 1200, there is no second birefringent walk-off plate betweenthe half-wave plates 510 and 511 and the lens 513. Instead, there is abirefringent wedge 1201 disposed between the lens 513 and the non-linearinterferometer 514. Furthermore, the separator 1200 is constructed suchthat signal light emanating from Fiber A 501 and Fiber B 502 passesthrough the center of lens 513 whereas signal light emanating from FiberC 503 and Fiber D 504 passes through the edge of lens 513.

The focusing properties of the lens in separator 1200 cause deflectionof sub-signal rays emanating from Fiber C 503 and Fiber D 504 in thefirst direction, that is, in the direction of the paths of thesub-signal rays emanating from Fiber A 501 and Fiber B 502. Afterpassing through lens 513, all sub-signals are intercepted by thebirefringent wedge 1201. Because of the double-refraction properties andorientation of the birefringent wedge 1201, this wedge refracts ordeflects the sub-signals comprised of horizontally polarized light—thosefrom fibers 503 and 504—to a greater extent than it refracts or deflectsthe sub-signals comprised of vertically polarized light—those fromfibers 501 and 502. The birefringent wedge 1201 is shaped and orientedsuch that the difference in deflection therein between the sub-signalsfrom fibers 501 and 502 and those from fibers 503 and 504 is equal andopposite to the lens-induced deflection of sub-signals from fibers 503and 504. The combination of the lens 513 and the wedge 1201 in thisfashion causes a superimposition of sub-signal images from fibers 501and 502 with those of fibers 503 and 504, respectively. Thesuperimposition is similar to that shown in cross section X-X′ 804 ofFIG. 8 or X-X′ 904 of FIG. 9, except that the sub-signal ray paths aredeflected slightly off the separator main axis—a line parallel to theinput and output fibers—by the wedge. For this latter reason, thenon-linear interferometer 514 is tilted, as shown in FIG. 12, by asimilar angle relative to its position in separator 500.

Aside from the means of superimposing sub-signal images, the operationof the second separator embodiment, separator 1200, is identical to thatof the first embodiment, separator 500, and will not be repeated here.The separator 1200 has the advantage that a birefringent wedge ofprecise thickness is not required to superimpose the various sub-signalimages. However, for proper operation, the deflections imposed by thelens 513 and the wedge 1201 must be precisely matched. This matchingcondition may be accomplished via slight adjustments to the position andtilt angle of the wedge 1201.

A third embodiment of the current invention is shown in FIG. 13 In theseparator 1300 shown in FIG. 13, the four-fiber ferrule 515, the fourfibers 501-504, the four collimator lenses 505-508, the firstbirefringent plate 509, the first and second 90° half-wave plates 510and 511, the lens 513 and the non-linear interferometer 514 are commonto the separator 500. However, in the separator 1300, there is only asingle half-wave plate 510, and there is no second birefringent walk-offplate between the half-wave plate 510 and the lens 513. Instead, thereis a prism 1301 and also a polarization beam splitter 1302 both disposedbetween the half-wave plate 510 and the lens 513. The half-wave plate510 is disposed so as to intercept only the four innermost raysemanating from or destined for the four input/output ports, as shown inFIG. 13. The prism 1301 is disposed so as to intercept the signalsemanating from Fiber C 503 and Fiber D 504 and turn or deflect theirdirections of propagation by 90°. This turning direction is in the firstdirection, that is, in the direction of signals emanating from Fiber A501 and fiber B 502 and such that the signals emanating from Fiber C 503and Fiber D 504 are subsequently intercepted by the polarization beamsplitter 1302. The polarization beam splitter is disposed such thatlight sub-signals emanating from Fiber A 501 and Fiber B 502 passtherethrough without deflection whilst sub-signals emanating from FiberC 503 and Fiber D 504, after having been deflected by the prism 1301,are deflected by 90′ by the polarization beam splitter. The combineddeflections by prism 1301 and polarization beam splitter 1302 upon thepropagation paths of signals emanating from Fiber C 503 and Fiber D 504are such that, subsequent to passage through polarization beam splitter1302, sub-signals from Fiber A 501 and Fiber B 502 are superimposed uponthose from Fiber C 503 and Fiber D 504, respectively.

The superimposition of sub-signals in the separator 1300 is identical tothat already described for separator 500 and is as exactly as shown inFIGS. 8 and 9 for sub-signals of odd and even channels, respectively.The operation of separator 1300 is similar to that shown in FIGS. 8 and9 and described in reference thereto except that the cross-section X-X′is disposed to the side of polarization beam splitter 1302 facing lens513, and the orientations of polarization planes of all sub-signalsdiffer by 90 degrees (relative to their respective orientations in theseparator 500) along the sub-signal pathways between the half-wave plate510 and the non-linear interferometer 514. The forward-propagatingpathways of odd-channel sub-signals C′ 815, D′ 817, C 814 and D 816 andof even-channel sub-signals C′ 915, D′ 917, C 914 and D 916 are alldeflected by 90′ by the prism 1301. Subsequently, these same sub-signalsare deflected by 90° by the polarization beam splitter 1302 such that,as shown in cross section X-X′ 804 of FIG. 8 and X-X′ 904 of FIG. 9, thesub-signals from Fiber A 501 and Fiber B 502 are superimposed upon thosefrom Fiber C 503 and Fiber D 504, respectively. The sub-signals C′ 815,D′ 817, C 814, D 816, C′ 915, D′ 917, C 914 and D 916 are allhorizontally polarized before entering prism 1301. After being deflectedby and exiting prism 1301, these sub-signals remain horizontallypolarized. The horizontal polarization of sub-signals C′ 815, D′ 817, C814, D 816, C′ 915, D′ 917, C 914 and D 916 comprises s-polarizationwith respect to the polarization beam splitter 1302 and thus the pathsof these sub-signals are deflected by 90° at the polarization beamsplitter 1302. The sub-signals A′ 811 B′ 813, A 810, B 812, A′ 911, B′913, A 910 and B 912 are all vertically polarized before enteringpolarization beam splitter 1302. This vertical polarization comprisesp-polarization with respect to the polarization beam splitter 1302 andthus these sub-signals are transmitted directly through beam splitter1302 without deflection. By this means, the sub-signals from Fiber A 501and Fiber B 502 become superimposed upon those from Fiber C 503 andFiber D 504, respectively. Other aspects of the operation of separator1300 are identical to those already described for separator 500 and willnot be repeated here.

The separator of the present invention has the advantage, relative toseparators of conventional wavelength division multiplexers, of a highertolerance to drifts due to the increase in the widths of the pass bandsprovided by the non-linear interferometers contained therein. It offersthe further advantage, relative to separators and dense wavelengthdivision multiplexers utilizing polarization beam splitters, of agreater efficiency of separation between the sets of separated channels.This latter advantage arises because physical separation of beam pathsis effected by birefringent walk-off plates, rather than polarizationbeam splitters. The efficiency of separation of an unpolarized lightbeam into a first light beam comprising a first plane polarizationdirection and a second light beam comprising a second plane polarizationdirection perpendicular to the first is greater for a birefringent platethan it is for a polarization beam splitter. That is, the degree ofisolation of a first set of signals from contamination from a second setof signals physically separated according to the differing planepolarization directions of the light rays of which they comprised ismuch greater for the separator of the present invention than it is forseparators based upon polarization beam splitters.

Another advantage of the separators of the present invention is theability to place them in a multi-stage parallel cascade configuration toreduce insertion loss as part of a larger dense wavelength divisionmultiplexer. This configuration is illustrated in FIG. 14 and has beendisclosed in co-pending U.S. Patent Application entitled “Fiber OpticDense Wavelength Division Multiplexer Utilizing a Multi-Stage ParallelCascade Method of Wavelength Separation,” Ser. No. 09/130,386, filed onAUGUST 6, 1998. Applicant hereby incorporates this application byreference. In FIG. 14, an optical signal containing channels λ₁-λ_(n)enters the dense wavelength division multiplexer 1400 of the presentinvention through Node A 240. The signal passes through a separator 1410A of the present invention. The separator 1410 A divides the signal intotwo separate signals, one containing the odd channels (λ₁, λ₃, λ₅ . . .) (1430) and the other containing the even channels (λ₂, λ₄, λ₆ . . . )(1440) as described above. These odd and even channels are each passedthrough another separator, 1410 B and 1410 C, respectively, whichfurther divides them by every other channel. The separator 1410 B, andspecifically the set of non-linear interferometers (not shown)comprising this separator, is modified so as to separate the set ofchannels λ₁, λ₅, λ₉ . . . from the set of channels λ₃, λ₇, λ₁₁ . . .through adjustment of the wavelength spacing of channels whosepolarization directions are rotated. Likewise, the separator 1410 C, andspecifically the set of non-linear interferometers comprising thisseparator, is modified so as to separate the set of channels λ₂, λ₆, λ₁₀. . . from the set of channels λ₄, λ₈, λ₁₂ . . . through a similaradjustment. Similar channel division continues until only one channel isoutputted to each optical fiber 250-1 through 250-n. The separators 1310A, 1310 B, and 1310 C may be of the first, second, or third embodimentsas described above.

Although the separator of the present invention has been described asbeing utilized with the multi-stage parallel cascade configuration ofthe present invention, one of ordinary skill in the art will understandthat the separator of the present invention may be utilized with otherconfigurations without departing from the spirit and scope of thepresent invention.

In a dense wavelength division multiplexer constructed utilizing thepresent invention in a multi-stage parallel cascade configuration, thereis no decrease in pass band widths relative to those of the channelseparator in the first stage. This is in contrast to and an advantageover conventional filter technologies which, when concatenated in seriesas part of a larger optical device, cause a decrease in overall passband width of the filter ensemble relative to any individual filter. Thedense wavelength division multiplexer of the present invention is thusfree of the increased insertion losses associated with such reduced passband widths.

An improved separation mechanism to be used in a dense wavelengthdivision multiplexer has been disclosed. The mechanism of separating oneor more of the plurality of optical channels in the present inventionincludes birefringent beam separation and recombination plates,opticalrotation elements, a lens and a non-linear interferometer. The presentinvention provides an ease in alignment and a higher tolerance to driftsdue to the increase in the widths of the pass bands and also provides animproved efficiency of separation of channels. It may also be easilymodified to perform the add/drop function as it separates channels. Thematerials required to manufacture and implement the dense wavelengthdivision multiplexer in accordance with the present invention arereadily available. The present invention thus does not require specialor expensive materials or processes. It is thus cost effective.

Although the present invention has been described in accordance with theembodiments shown, one of ordinary skill in the art will readilyrecognize that there could be variations to the embodiments and thosevariations would be within the spirit and scope of the presentinvention. Accordingly, many modifications may be made by one ofordinary skill in the art without departing from the spirit and scope ofthe appended claims.

What is claimed is:
 1. A dense wavelength division multiplexer forseparating an optical signal into optical channels, comprising: meansfor inputting an optical signal, the optical signal comprising aplurality of optical channels; birefringent plate optically coupled tothe inputting means and an outputting means; a first and secondhalf-wave plates, each partially optically coupled to the birefringentplate on a side opposite than the inputting and outputting means, suchthat the first and second half-wave plates each is capable ofintercepting approximately one-half of a signal from the birefringentplate; a lens optically coupled to the birefringent plate and to thefirst and second half-wave plates on a side opposite to the birefringentplate; a birefringent wedge optically coupled to the lens on a sideopposite to the first and second half-wave plates; and a non-linearinterferometer optically coupled to the birefringent wedge on a sideopposite to the lens; and means for outputting the separated pluralityof optical channels along a plurality of optical paths.
 2. The densewavelength division multiplexer of claim 1, wherein the inputting meanscomprises: it at least one lens optically coupled to the birefringentplate; and at least one optic fiber optically coupled to the at leastone lens.
 3. The dense wavelength division multiplexer of claim 1,wherein the outputting means comprises: at least one lens opticallycoupled to the birefringent plate; and at least two optic fibersoptically coupled to the at least one lens.
 4. The dense wavelengthdivision multiplexer of claim 1, wherein the lens is disposed such thata signal passing through the lens from the inputting means and a signalpassing through the lens towards the outputting means are notequidistant from the center of the lens.
 5. The dense wavelengthdivision multiplexer of claim 1, wherein the birefringent wedge isshaped and oriented such that a deflection of signals passing through itis equal and opposite to a deflection induced by the lens.
 6. The densewavelength division multiplexer of claim 1, wherein the nonlinearinterferometer comprises: a first glass plate optically coupled to asecond glass plate, forming a cavity; a first reflective coatingresiding inside the cavity and on the second glass plate; a secondreflective coating residing inside the cavity and on the first glassplate; a first waveplate residing inside the cavity between the firstand second glass plates; and a second waveplate residing outside thecavity and optically coupled to the first glass plate.
 7. The densewavelength division multiplexer of claim 6, wherein the first reflectivecoating comprises a reflective coating with a reflectivity of 100%. 8.The dense wavelength division multiplexer of claim 6, wherein the secondreflective coating comprises a reflective coating with a reflectivity ofapproximately 18%.
 9. The dense wavelength division multiplexer of claim6, wherein the first waveplate comprises a λ/4 waveplate.
 10. The densewavelength division multiplexer of claim 6, wherein the second waveplatecomprises a λ/8 waveplate.